1. Field of the Invention
The present invention relates generally to alignment targets or marks for use in photo lithography masking operations of integrated circuit (IC) chips and more specifically to alignment targets having enhanced contrast.
2. Description of the Related Art
The surface of the IC wafer during Back End Of the Line (BEOL) processing does not tend to have great reflectivity contrast between components (e.g., wiring levels). In contrast, during Front End Of the Line (FEOL) processing, the base material does provide reflectivity. In particular, at the BEOL stage, during semiconductor processing, wiring levels are separated by thick layers of insulating materials. Each wiring level is a reflective film and the insulating films are usually highly transparent at the alignment wavelength. To align to these wiring levels, alignment marks or targets are placed in the IC wafer to provide reflectivity contrast. Alignment targets generally include a series of indicia on the surface or subsurface of a substrate which are used to align masking patterns for photo lithographic machinery. The alignment target is used for not only locating a particular position on the wafer, but is also used to align the wafer in two axes. The degree of accuracy of the alignment is a function of the resolution and contrast of the optical system. As IC chips become smaller and smaller, the challenge for alignment targets having adequate contrast for resolution increases.
Two alignment systems used to provide reflectivity contrast are a bright field alignment system and a dark field alignment system. In a bright field alignment system, contrast is seen between the light reflecting off of the metal wiring level's alignment mark and the background light reflected off of underlying reflective films (usually the silicon wafer itself). The metal can be from 1 to 7 .mu.m above these underlying films making it difficult to focus both simultaneously. The thickness of the insulating film on top of the mark (and the photo resist on top of that) determines whether the reflection will be dark or light due to the thin film interference effect. Variations in the thickness of this insulating layer driven by the deposited insulator as well as the chemical mechanical polish planarization (or other planarization techniques) can cause the mark to be light or dark within a wafer. This makes the algorithm that detects the mark having a resultant signal that fails. This variation in insulator thickness is also apparent in the background signal as reflected back out of focus from the underlying reflector. Because of these problems, bright field marks are difficult to use for alignment.
A dark field alignment mark cancels out the directly reflected light coming from edges of an alignment mark. Thus, a strong reflectivity contrast signal can be seen from the edges of the mark. A difficulty in dark field alignment is that process variations cause changes in the slopes of metal wires across a mark causing an apparent shift in the detected mark position. The problem of these process variations are due to etch variations caused by pattern density differences across the wafer and even within the mark itself. The etch process for the metal lines erodes the photo resist defining the lines at rates determined by pattern density thus changing the slope of the metal lines while they remain the correct dimension at the bottom.
Heretofore, various techniques have been used to enhance the contrast of alignment marks on IC wafers to improve resolution and accuracy. U.S. Pat. No. 4,632,557, entitled ALIGNMENT TARGET IMAGE ENHANCEMENT FOR MICROLITHOGRAPHY PROCESS and assigned to Harris Corporation, discloses an alignment target having a reflective region surrounded by a light scattering region on a substrate to increase contrast. The width of the reflective portion of the light scattering region is substantially smaller than the width of the target region along the orthogonal axis of the substrate. The light scattering region includes a plurality of peaks and valleys having parallel axes which are oblique to the axis of the substrate.
Japanese publication No. 62-025415, entitled ALIGNMENT MARK AND MANUFACTURE THEREOF, published on Feb. 3, 1987 and assigned to Cannon Inc., discloses an alignment mark having differing heights due to two patterns which cross one another (FIG. 3c). The first pattern is formed by PSG (polysilicon glass) and is square shaped. The first pattern crosses a second pattern formed by a nitride film and having a plurality of valleys therein. The interference of light differentiates due to the height of the steps of the two patterns crossing one another.
Other patents which disclose various types of alignment marks include: U.S. Pat. No. 5,334,466, entitled X-RAY MASK AND PROCESS COMPRISING CONVEX-CONCAVE ALIGNMENT MARK WITH ALIGNMENT REFLECTION FILM, assigned to Matsushita Electric Industrial Co.; U.S. Pat. No. 4,986,637, entitled METHOD OF MANUFACTURE TIW ALIGNMENT MARK AND IMPLANT MASK, assigned to Raytheon Company; and U.S. Pat. No. 5,264,310, entitled ALIGNMENT MARK HAVING REGIONS FORMED BY ENERGY BEAM IRRADIATION THEREIN AND METHOD OF MANUFACTURING THE SAME, assigned to Mitsubishi Denki Kabushiki Kaisha. All the references cited herein are hereby incorporated by reference. The summary of each reference described herein should not be relied upon or substituted for a thorough reading of each individual reference. None of the aforementioned difficulties of detecting a marks position in dark field alignment have been adequately addressed by the above references.